Researchers found that overweight people repeatedly placed in a stressful situation exhibited increasing amounts of interleukin-6, a protein that promotes inflammation in the body, in their blood. Normal weight people did not exhibit this escalation in interleukin-6 levels when exposed to repeated stress.

The inflammation caused by interleukin-6 has been associated with a number of conditions for which obesity itself creates an increased risk, including hardening of the arteries, type 2 diabetes, cancer and fatty liver disease, the researchers said.

"You already are at risk for these diseases by being obese, and then you have these magnified responses that further exacerbate your risk," said study author Christine McInnis, a Ph.D. student in Brandeis University's Laboratory for Biological Health Psychology, in Waltham, Mass.

On two consecutive days, researchers placed people of various body sizes in stressful situations, including a very unfriendly job interview and a difficult oral math exercise, McInnis said. They then took blood samples to see how the stress affected the person's body chemistry.

Lean people started out with lower interleukin-6 levels than obese people, but both lean and obese participants exhibited similar amounts of biochemical response to stress on the first day, the investigators found.

However, overweight or obese individuals exhibited an interleukin-6 response on the second day nearly double that of their response the day before. By comparison, the second-day response of lean people was the same as it was their first day.

This indicates that obese people are physically affected by repeated stress much more dramatically than people at normal weight, and recover from stress more slowly, McInnis said.

Further, the relationship between body mass index (a measurement of body fat based on height and weight) and interleukin-6 levels was linear, the study authors reported.

"When we brought all these folks back in for the second day, we found the greater the body fat someone had, the larger the interleukin-6 response they had," she said. "It seems that every percentage point of body fat makes you more susceptible to inflammation."

The findings "suggest a possible explanation for the increased risk of illness and disease in overweight and obese individuals," said Christopher Ochner, an obesity and nutrition expert at Mount Sinai Hospital in New York City, who was not involved with the study. "There are almost certainly many pieces to that puzzle, but this may indeed be an important one."

Ochner noted that the study did not include obese people with a BMI greater than 35, which may have caused researchers to slightly understate the impact that stress can have on people carrying a lot of extra weight.

"It is possible, if not likely, that their results would have been even more robust were clinically severely obese individuals included," Ochner said.

The findings were published online recently in the journal Brain, Behavior and Immunity.